A basic GUI application functions by running an endless event, which
handles each mouse movement, button click or keystroke. GTK+
simplifies this procedure by taking a step away from the event loop
and defining a set of "signals", which more closely matches an event
which has taken place to the object that that event corresponds to.
Even the event loop is abstracted away from the user, by simply
calling the function
gtk_main();
which starts up the event loop, the details of which are hidden from
the user.
In the case of a button in a window, for instance, the separate mouse movement events are replaced by a single "enter" signal, indicating the mouse has moved over the button. The event that says a mouse button was pressed is replaced by a signal that the button (the object in the window) was clicked.
Signals are processed by attaching a "callback" function to a given signal for any particular object. For example, after creating a new button,
button = gtk_button_new();
gtk_signal_connect (GTK_OBJECT (button), "clicked",
GTK_SIGNAL_FUNC (ButtonPressed), NULL);
which connects the function "ButtonPressed" to the "clicked" signal
that is generated when the button is pressed.
The callback function, "ButtonPressed" in this case, may be defined as
void ButtonPressed( GtkWidget *widget, gpointer data )
{
g_print ("Button was pressed!\n");
}
Note that the callback function has two arguments. The first
identifies which object the signal was generated for, in this case, the
button. The second argument is a pointer to any further data supplied
to the callback function at the time the function was connected to the
signal. In the example above, NULL was given, meaning there is in fact no
extra data for this function.
Note that this function does not know which mouse button (if any) was used to click the application's button. In order to obtain this more detailed information, a different signal may be used, in this case the signal called "button_press_event" which refers to the actual event corresponding to the pressing of the mouse button. The callback function in this instance,
ButtonPressed_event(GtkWidget *widget,
GdkEventButton *event, gpointer data)
has an extra argument, a pointer to an "event" structure, which among
other things identifies which mouse button was actually pressed.